<template>
  <!-- 回到头部 -->
  <div class="scroll_top" :class="btnFlag ? 'active' : ''" @click="backTop"></div>
</template>

<script>
export default {
  name: 'footer',
  components: {},
  data() {
    return {
      scrollTop: null,
      btnFlag: false,
    };
  },
  computed:{
  },
  mounted() {
    window.addEventListener('scroll', this.scrollToTop);
  },
  destroyed() {
    window.removeEventListener('scroll', this.scrollToTop);
  },
  methods: {
    // 点击图片回到顶部方法，加计时器是为了过渡顺滑
    backTop() {
      const that = this;
      let timer = setInterval(() => {
        let ispeed = Math.floor(-that.scrollTop / 5);
        document.documentElement.scrollTop = document.body.scrollTop = that.scrollTop + ispeed;
        if (that.scrollTop === 0) {
          clearInterval(timer);
        }
      }, 16);
    },
    // 为了计算距离顶部的高度，当高度大于60显示回顶部图标，小于60则隐藏
    scrollToTop() {
      const that = this;
      let scrollTop = window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop;
      that.scrollTop = scrollTop;
      if (that.scrollTop > 0) {
        that.btnFlag = true;
      } else {
        that.btnFlag = false;
      }
    }
  }
};
</script>

<style lang="scss" scoped>
.scroll_top {
  right: 57px;
  bottom: 50px;
  width: 50px;
  height: 0px;
  position: fixed;
  visibility: 1;
  opacity: 1;
  background-image: url(../../assets/scroll.png);
  cursor: pointer;
  transition: 0.8s;
}
.active {
  height: 50px;
}
.scroll_top:hover {
  opacity: 0.2;
}
</style>
